The electrochromic systems that are the object of this invention consist of a stack of thin layers deposited on a transparent substrate such as a glass sheet, a second substrate--itself also transparent in the case of a system operating by transmission--preferably covering the unit. This stack comprises at least one layer of an electrochromic material that can insert lithium cations, protons or ions generally in a reversible way and whose inserted or removed states exhibit different colorations.
The most used electrochromic materials are transition metal oxides whose change of coloration is due to an electrochemical process of insertion of the cation leading to a change of the degree of oxidation of the metal. These materials are divided into two groups, those with cathodic coloration which take on color during an insertion such as the oxides of tungsten, molybdenum, niobium, titanium and those with anodic coloration which are bleached during an insertion such as the oxides of nickel, iridium, rhodium or cobalt.
So that the electrochemical reaction occurs, it is necessary that the stack comprises on one side of the layer of electrochromic material a source of cations, consisting of a layer of an ionic conductive electrolyte and on the other side a source of electrons consisting of an electroconductive layer. Further, it is well known that behind the ionic conductive electrolyte, it is very advantageous to provide a counter electrode able to receive the cations released during the removal phase then to restore them in insertion phase. This counter electrode should, moreover, in a system operating by transmission, be in a colorless or bleached state when the main electrode is itself in the bleached state. it is easily seen that this role can be more particularly held by an electrochromic material having an inverse operation relative to that of the main layer; the most ordinary electrochromic systems are therefore primarily based on pairs of cathode and anode electrochromic materials, the term of counter electrode being reserved for the less high-performing of these materials from the standpoint of the coloration intensity obtained.
In U.S. Pat. No. 4,664,934, a system based on the reversible insertion of protons, with a pair of tungsten oxide/iridium oxide electrochromic materials has thus been proposed. This same pair has also been used for systems based on the reversible insertion of lithium ions. The selection of iridium oxide as anode electrochromic material is linked to its good resistance to corrosion in acid or basic medium and to the fact that its coloration which tends toward brown does not interfere too greatly with the midnight blue coloration of the tungsten oxide layer.
Nevertheless, the iridium oxide has the defect of exhibiting a residual coloration in its so-called bleached state, which is particularly troublesome in a system operating by transmission, such as, for example, a window.